Sanford, a Republican who admitted an extramarital affair in 2009, was ready to quit politics for good if he was not victorious in South Carolina's 1st Congressional District.

He was the favorite in the GOP stronghold, which he represented from 1995 to 2001, but the revelation that his ex-wife accused him of trespassing at her home caused the National Republican Congressional Committee to withdraw its financial and logistical support. Colbert Busch, a businesswoman and sister of comedian Stephen Colbert, stayed competitive in the race and was leading a final Public Policy Polling survey two days before the special election.

If he loses the special election in South Carolina's 1st Congressional District to Democrat Elizabeth Colbert Busch, the Republican who was once touted as a possible presidential candidate says he won't run for office again.

"I think you can go back in and you can ask for a second chance in a political sense once," Sanford told the Associated Press and other news outlets. "I've done that, and we'll see what the voters say."

Voters are deciding whether they will be represented by Sanford, who was their congressman from 1995 to 2001, or Colbert Busch, a political neophyte and sister of comedian Stephen Colbert. Mitt Romney carried the district by 18 percentage points in the 2012 presidential election.

Businesswoman Colbert Busch was confident after voting. "I obviously feel very positive, very encouraged. We are all very excited," she said. "I'm predicting victory."

Turnout was expected to be light Chris Whitmire, a spokesman for the South Carolina Election Commission, said 15.6% of the state's eligible voters cast ballots in the GOP and Democratic primaries.

A survey by Public Policy Polling released Sunday night showed the race is in a dead heat, with Sanford leading Colbert Busch by 1 percentage point.

Sanford is on the ballot for the first time since he admitted in 2009 that he had an extramarital affair with Maria Belen Chapur, now his fiancée. His excuse that he was hiking on the Appalachian Trail - when he was actually in Argentina visiting his mistress - became a punch line on late-night TV.

Sanford was a favorite to win until it was revealed that his ex-wife, Jenny, accused him of trespassing at her house in violation of their divorce decree. He has said he was there to watch the Super Bowl with one of the couple's sons.

"I absolutely failed. Period," Sanford said in an interview on CNN, one of several he had on Election Day. Earlier, he told MSNBC that "one event does not define your life."

Still, Sanford campaigned in the final weeks without the financial or logistical help from the National Republican Congressional Committee, which withdrew after the trespassing charge became public. Sanford has stressed Colbert Busch has received help from labor unions and Democratic groups from outside the district, and tied her to House Democratic leader Nancy Pelosi.

"Our race has been officially Pelosi-ized," said Chad Connelly, chairman of the South Carolina Republican Party. "This has been a Republican seat for 30 years. Our people don't want to give a seat to a progressive."

Colbert Busch made one reference to Sanford's affair during their lone debate and has primarily focused on her plans to create jobs. She is the business development officer at Clemson University's Restoration Institute and has worked in the shipping industry.

On the campaign's final weekend, Colbert Busch was endorsed by The Post and Courier. The Charleston newspaper said she would bring "a fresh approach" to the gridlock in Washington and said she would be a "balanced alternative" to Sanford.

The House Majority PAC, which seeks to elect Democrats, has spent $450,000 to help Colbert Busch. The super PAC has paid for three TV ads and five direct-mail pieces to remind voters, especially women, of Sanford's voting record and extramarital affair.

"It's clearly a very Republican district. The fact that it is a competitive race shows how flawed a candidate Mark Sanford is," said Andy Stone, a spokesman for House Majority PAC. "As Republicans choose flawed candidates like him, Democrats can make these races competitive."

Since 2008, Democrats have won several special elections in GOP-held House districts in Illinois, Louisiana, Mississippi and New York. Connelly, the state GOP chairman, played down the outside spending by Democrats and sounded confident the "boots on the ground" operation that Republicans have in place will push Sanford across the finish line.